Many industrial processes require utilization of large amounts of water. In order to recycle this waste water, it must be treated before it can be reused. Effective recycling nay include the removal of turbidity-causing components from the waste water.
Turbidity-causing components can be any organic or inorganic species of sufficient particle or droplet size to create a turbid, light-dispersed appearance in waste waters, industrial waters, or natural waters. These turbidity-causing components can be of an inorganic nature, an organic nature, a pigment dispersion, a colloidal humic material suspension, sewage components, or admixtures of any of the above in any ratio in waters of any description thereby obtaining a turbid translucent effect.
Turbid waters are any type of waters having dispersed therein any type of turbidity-causing component, as described above, or any other type of turbidity-causing component which might be similar in effect when dispersed in these types of waters. For example, colloidal matter of either organic or inorganic or even mixed organic and inorganic character can be turbidity-causing. Such components may vary greatly in nature and concentration, but generally contain a highly anionic surface charge which causes the colloidal particles to repel one another, thus remaining dispersed in the water, rather than settling out in a reasonable time frame.
Examples of turbid waters include waste waters expelled from hydrocarbon processing plants, waste waters expelled from chemical processing plants which synthesize various types of rubber latexes, and effluent waters expelled from various kinds of processing plants using materials containing emulsified components which are of a hydrocarbon nature. Further, the waste waters may be from automotive plants or machining plant operations.
A common method for removal of suspended solids from various types of turbid waters is by the well-known chemical process of clarification. The primary unit operations in clarification are coagulation combined with or followed by flocculation. Coagulation is defined as destabilization of the solids through neutralization of the surface charge on particles or droplets dispersed in the water. Flocculation is defined as the agglomeration or coalescence of small particles or droplets into larger particles or droplets usually through bridging, generally called floc.
A method of clarification usually comprises addition to the aqueous solution to be treated of certain chemical additives known as coagulants, mixing of the chemical additive and aqueous liquid whereby floc formation occurs, and subsequent removal of this floc by a variety of methods. In some cases, the addition of a second higher molecular weight water-soluble polymer, generally known as a flocculant may aid in the formation of floc. The removal of floc may be achieved by mechanical separation means or by merely allowing forces of gravity to work upon the agglomerated solids whereby a removable sludge layer is created.
Among effective coagulants for this purpose are water-soluble cationic polymers. These materials function by neutralizing the charge on the surface of the dispersed particles or droplets which allow the turbidity-causing materials present in turbid waters to agglomerate or coalesce, and to settle, or float to the top of the aqueous medium where they are collected and removed by techniques familiar to the those skilled in the art.
For an example of effective coagulants, water-soluble cationic polymers in conjunction with polynuclear aluminum species for clarifying waters containing turbidity-causing components are disclosed in U.S. Pat. No. 4,655,934. Another example which discloses the reaction products of phenol, formaldehyde, and low molecular weight polyamines useful for removing turbidity from low turbidity industrial waste waters is disclosed in U.S. Pat. No. 4,308,149. However, more efficient processes for the removal of turbidity would represent an improvement over the prior art.
In addition to the removal of turbidity-causing components from waste water, many industrial processes necessitate removal of color before recycling of the water. Particularly, color removal from the effluent streams of paper mills presents a problem within the pulp and paper industry. It is necessary that these waste waters be treated for color removal prior to discharge.
The United States wood pulp production capacity is approximately 60 million tons per year. Since the average cellulose content of wood is about 40%, 150 million tons of wood are needed to produce this 60 million tons of pulp. The difference between these two numbers represents the lignin and hemicellulose which must be removed or separated in the pulping process in order to free the cellulose fibers.
The pulping process, however, does not remove 100% of the lignin present in the wood, with approximately 5% remaining after either Kraft or sulfite pulping (for mechanical pulping the amount is considerably higher). If a high grade paper is the desired end product, then this 5% residual lignin must be removed by bleaching the pulp.
Since over 35% of the pulp produced in the United States is bleached, there are about one million tons of lignin removed each year at the bleach plant, most of this in the caustic extraction stage. This number is significant because in the removal process (i.e., bleaching), most of this residual lignin is solubilized. This solubilized lignin is a strong absorber of visible radiation resulting from the conjugation of unsaturated and quinodal moieties formed during the oxidation step in the bleach plant. Consequently, the bleach plant effluent is highly colored. Although there are other sources of color in paper mill waste effluent, it is readily apparent that where bleaching is performed, its effluent can be expected to be the major contributor of waste color. Indeed, at Kraft bleach mills, the effluent from the first caustic extraction stage accounts for at least 70% of the waste color.
The goal of the pulping and bleaching operation is the removal of lignin and hemicellulose from the cellulose fiber in wood. The 95% that is removed by pulping is often burned as fuel in the process of recovering the inorganic chemicals present in the black liquor. In the bleaching operation, the 5% residual lignin is separated from the fibers by degradation and solubilization and ends up in the waste water. Chemical removal can therefore only be accomplished by reducing this solubility, which has proved to be a difficult task.
Therefore, the primary source of color in pulp is lignin. It has also been suggested that Kraft color is due to keto-enols produced from carbohydrates during the Kraft cook stage in the papermaking process. Chlorination of the pulp during the bleaching operation results in the formation of color bodies which are leached from the pulp by caustic alkali solutions. Thus, the caustic extract effluent contains a major proportion of the color bodies and the other organic materials which have to be disposed of during the waste water treatment.
The process of color removal from the effluent stream is further complicated by the presence of lime, solid particulate matter like pulp, clay, dispersant/surface active materials and polymers used during various stages in the papermaking process. The solid particulate matter is commonly referred to as anionic trash.
Most governmental regulations pertaining to color removal from the effluent stream of a papermaking process are directed to true color, i.e., platinum cobalt (Pt-Co) color using a DR2000 spectrophotometer). Nevertheless, there is increasing pressure on pulp and paper mills to lower the apparent color of the effluent water because that is the color visible to the naked eye. There are occasions when the true color of a system that has undergone treatment is low, but the corresponding apparent color is high. This problem is commonly caused by the presence of suspended particulate matter that causes an increase in the turbidity of the system. Therefore, it is important that any new treatment for color removal should not only remove the true color of the effluent, but should also lower the apparent color as well.
The pressure to remove color comes primarily from state environmental agencies. Previously, it was thought that the discharge of colored waste affected only the aesthetic value of the receiving body of water; however, biologists are becoming increasingly concerned about possible toxic effects, the effect of reduced light transmittance through the water causing reduced levels of photosynthetic activity, and of course, the resultant drop in dissolved oxygen concentration because of this drop in activity. Furthermore, although these colored, waste products are fairly refractory towards biological oxidation and since they become degraded in the aquatic environment, the oxidation products may be potentially harmful.
It has been shown that by-products are water soluble, and that a significant amount are produced. This puts severe demands on chemicals to be used for color removal. There are techniques already available, however, that can remove greater than 90% of the color from either total mill effluent or isolated waste streams, such as from the caustic extraction stage of the bleach plant. These techniques include chemical (e.g., alum, ferric, lime or polyelectrolytes), biological (e.g., white rot fungus) and physical processes (e.g., ultrafiltration, ion exchange and carbon absorption). However, none of these techniques enjoys widespread use due to prohibitive cost.
Chemical techniques for the removal of color include a decolorizing composition consisting of ferrous sulfate and a water-soluble cationic copolymer of epichlorohydrin and dimethylamine as disclosed in U.S. Pat. No. 5,200,089. Another example of a chemical treatment for the removal of color is a copolymer comprising diallyldimethyl ammonium chloride and a hydrophobic monomer selected from the group consisting of quaternized dimethylaminoethylacrylates and quaternized dimethylaminoethylmethacrylates as disclosed in U.S. Pat. Nos. 5,338,816; 5,283,306; 5,292,793 and 5,314,627.
The demands on a product used in a color removal application are quite severe, i.e., the product must be capable of reacting with the color bodies in a manner which results in their becoming insoluble and, because of the extremely large amount produced, the color removal product must work at very low weight ratios relative to the organic being removed or its use will be precluded by prohibitive costs.